Figure 1-4: Staircase effect in a raster-scan system
Raster-scan systems also have limitations in implementing animation. Two factors contribute to
this problem: first, all the screen pixels within a rectangular area must be updated with each image
change. Second, the successive images that create the illusion of motion must be flashed on the
screen at a fast rate to ensure smoothness. These constraints place a large processing load on
the microprocessor and the display system hardware, in any display system.
PC video technologies
In 1981 IBM introduced the first model of its microcomputer line. The original machines were
offered with either a monochrome display adapter (MDA), or a graphics system named the
color/graphics monitor adapter (CGA). IBM's notion was that users who intended to use the PC for
text operations exclusively would purchase a machine equipped with the MDA video system, while
those requiring graphics would buy one equipped with the CGA card. In reality, the CGA video
system provided only the most simple and unsophisticated graphics functions. The color card was
also plagued with interference problems that created a screen disturbance called "snow."
However, the fact that the original IBM Personal Computer was furnished with the optional CGA
card signaled that IBM considered video graphics an important part of microcomputing.
At present, PC video hardware includes an assortment of systems, cards, monitors, and options
manufactured and marketed by many companies. In the following pages we briefly discuss the
history and evolution of the better-known PC video systems. Systems that were short-lived or that
gained little popularity, such as the PCJr, the IBM Professional Graphics Controller, the Multicolor
Graphics Array, and the IBM Image Adapter A, are not discussed.
Monochrome Display Adapter
The MDA is the original alphanumeric display card designed and distributed by IBM for the
Personal Computer. Originally, it was sold as the Monochrome Display and Printer Adapter
because it included a parallel printer port. It could display the entire range of alphanumeric and
graphic characters in the IBM character set, but did not provide graphics functions. The MDA was
compatible with the IBM PC, PC XT, and PC AT, and some of the earlier models of the PS/2 line. It
could not be used in the PCJr, the PC Convertible, or MicroChannel PS/2 machines. The MDA
required a monochrome monitor of long-persistence (P39) phosphor. The video hardware was
based on the Motorola 6845 CRT controller. The system contained 4K of on-board video memory,
mapped to physical address B0000H.
The MDA is a pure alphanumeric display; the programmer cannot access the individual screen
pixels, but sees the video screen as a collection of character and attribute cells. The character
codes occupy the even-numbered bytes of adapter memory and the display attributes the odd-
numbered bytes. This special storage and display scheme was conceived to save memory space
and to simplify programming. Figure 1-5 shows the cell structure of the MDA video memory space
and the bitmap for the attribute cells.
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